This disclosure relates generally to the field of image processing and, more particularly, to techniques for modifying arbitrary transfer functions (tone curves) so as to effect a desired image enhancement.
Image enhancement methods may be made in either the spatial domain or the frequency domain. Spatial domain approaches perform direct manipulation of image pixel values. Frequency domain approaches modify the image's Fourier transform. In the spatial domain, many image enhancement operations can be implemented as, or approximated by, a transfer function. In general, a transfer function may be represented as any curve that can be drawn on a graph that relates an input image quantity (e.g., brightness, color, hue, saturation) to its output image counterpart (e.g., brightness, color, hue, saturation).
One frequently used image enhancement operation is one in which an image's detail is improved. In Dynamic Range Compression (DRC) operations for example, a transfer function may be used to map a large number of brightness values in an input image into a smaller number of brightness values in an output image for display. As a side-effect, such operations can often result in a flat and visually unappealing image where fine detail is muted. It would therefore be beneficial to provide a mechanism to after an initial transfer function (aka, a tone curve) in a manner that preserves the image's overall visual appearance but improves its fine detail.